As someone teaching and researching optics for essentially my entire professional life, I came to Sommerfeld's book relatively late. There is of course a lot of overlap with standard, and later texts, and in particular, the community consciousness has been dominated by the encyclopedic, but not necessarily reader friendly, Born and Wolf. This is in stark contrast to Sommerfeld's pedagogy, gently reminding the reader even of sign conventions. However, there is one topic in which the treatment here is, in my opinion, unsurpassed, and that is what should be the simplest exercise in wave optics beyond the single plane wave - diffraction by a straight edge. If you thought that was a trivial application of Fresnel integrals, go to pages 258-265, to both understand the physics and witness the mathematical power. Historically, one could call it the first rigorous, honest solution of a diffraction problem, which had to wait till 1897, Sommerfeld himself, of cousre, Not that the other pages are to be ignored - and some of them plunge into a level of detail regarding experiment which would surpise anyone who thought Sommerfeld's physics was merely mathematical. The same applies to all the other volumes of the series, of course, but I'm singling this out, having just dipped into the book again, courtesy the internet archive - my own copy was lent out long ago, and hopefully is enlightening someone somewhere on optics